1. Field of the Invention
The present invention relates to vacuum pressure swing absorption systems for selectively absorbing nitrogen from air to produce an oxygen-enriched gas stream and, in particular, to an apparatus and method for operating such a system using a multiport valve.
2. Description of Related Art
The separation of air by a vacuum pressure swing absorption (VPSA) system to produce oxygen or nitrogen has become economically attractive recently. The cycles employed by this type of system generally have relatively short times. Improvements in the VPSA system have involved the reduction of the time for each of the steps. This reduction in time has made the operation of process valves quite critical in the successful operation of new cycles. The time required to open and close the process valves has been found to be an important factor. Automatic valves currently available for this type of service are limited in their ability to respond to cycle requirements.
Numerous VPSA systems are in use today. These systems generally employ a multiplicity of separate double-ported valves to direct the flow of fluids in accordance with the specific cycle. These valves are interconnected by a complex and expensive piping system. As a result, the associated capital cost of these VPSA systems is high, and system optimization is limited by the physical characteristics of the valves, resulting in high operating costs.
For example, U.S. Pat. No. 5,549,733 discloses a vacuum-type cycle used for the production of oxygen from air in which the equipment includes six individual automatic valves to control the cycle. In another case, U.S. Pat. No. 5,223,004, a method of absorption separation of gases of the kind which regenerates the absorbent by pressure variation and uses two absorbers also incorporates at least six automatic valves to control the flow, pressures and timing of the cycle. A process for oxygen enrichment of air by means of a vacuum swing absorption system in a three-bed process disclosed in U.S. Pat. No. 5,144,440 uses a number of individual automatic valves. A valve assembly having two valve members which use rotating plugs for directing flow is disclosed in U.S. Pat. No. 5,114,441 for an oxygen concentrator system. A two-bed pressure swing process disclosed in U.S. Pat. No. 5,122,164 uses six automatic valves for controlling the cycle along with two automatic valves for equalization steps. Thus it would be desirable to eliminate all or most of the valve skid bottom piping and switching valves associated with the feed and vacuum systems in a VPSA system.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a system for selectively removing a desired component, for example nitrogen, from a fluid such as air which utilizes fewer valves.
It is another object of the present invention to provide such a system which further eliminates excess piping connecting the system components.
A further object of the invention is to provide a VPSA system with improved cycle times and efficiencies.
It is yet another object of the present invention to provide a VPSA system which uses less power.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.